1. Field of the Invention
The present invention relates to a coaxial two-wheel vehicle including two wheels disposed on the same axis center line, particularly relates to a coaxial two-wheel vehicle which can travel freely with a person riding thereon.
2. Description of the Related Art
Japanese Published Patent Application No. 04-201793 discloses a coaxial two-wheel vehicle of this kind in related art, for example. This patent reference discloses a travel controller for an unstable vehicle, which enables an easily falling unstable vehicle to travel stably. The travel controller described in this patent reference is an apparatus controlling a travel of an unstable vehicle which includes at least two wheels disposed at other positions than a position on the same straight line parallel with a forwarding direction and a seat fixed on a wheel shaft connecting those wheels and in which the center of gravity is positioned above a rotational axis line of those wheels. Further, this apparatus includes a detector, a driver and a control value decider. The detector detects a tilt angle and/or a tilt angular velocity of a line connecting the rotational axis line and the center-of-gravity position with respect to a gravitational direction orthogonal to the wheel shaft. The driver is fixed to the wheels to drive the wheels. The control value decider decides a control value of this driver such that the above-described detection value is reduced.
According to the travel controller for an unstable vehicle having such configuration described in the above patent reference, it is expected to have such effectiveness that the vehicle is inclined in a desired back and forth direction and so the vehicle can travel in that desired direction. In addition, since only two wheels contact the ground, the vehicle is capable of making a small turn and of traveling through a narrow place without difficulty (refer to a paragraph regarding “The Effect of The Invention”).
Japanese Published Patent Application No. 2005-6436 discloses another coaxial two-wheel vehicle in related art, for example. This patent reference discloses a coaxial two-wheel vehicle including wheels at both ends on the same axle. The coaxial two-wheel vehicle described in this patent reference includes a pair of wheels, a wheel axle, a base, a pair of drive motors and a controller. The wheel axle is installed between the pair of wheels. The base is supported on the wheel axle in a manner capable of moving on a tilt. The pair of drive motors are mounted on the base in order to drive each of the pair of wheels. The controller sends an operation command to the pair of drive motors. The base includes an acceleration detector detecting acceleration in the vertical direction, and the controller sends the operation command to decelerate the vehicle to each of the pair of drive motors when an absolute value of acceleration detected by the acceleration detector during a travel is equal to or more than a predetermined threshold value.
According to the coaxial two-wheel vehicle of the above Japanese Published Patent Application No. 2005-6436 having such configuration, it is expected to have such effectiveness as in the followings. The coaxial two-wheel vehicle includes the acceleration detector to detect the acceleration in the vertical direction. Therefore, in the case where the absolute value of acceleration detected by the acceleration detector during the travel is equal to or more than the predetermined threshold value such as the case of the vehicle running on a step, for example, the vehicle can travel safely by following the step or the like, because the detector sends the operation command of deceleration to each of the pair of drive motors.
Further, Specification of U.S. Pat. No. 5,791,425 discloses further another coaxial two-wheel vehicle in related art. This patent reference discloses a transportation vehicle with a loop control. The transportation vehicle described in this patent reference includes a platform for a rider to ride on, right and left wheel groups, a pair of wheel drivers, a handle, and the like. The right and left wheel groups each have a plurality of wheels disposed in a direction orthogonal to a traveling direction of the platform. The pair of wheel drivers drive and rotate the right and left wheel groups individually. The handle with a grip on the top end is installed upright on the upper surface of the platform.
However, in each of the coaxial two-wheel vehicles described in the above-described patent references, the handle is installed upright by fixing the bottom end thereof to a step plate (a frame in the first patent reference, a base in the second patent reference, and a platform in the third patent reference) as a riding portion for a person to ride on. Further, a support portion supporting the wheels in a freely rotatable manner (a DC servo motor in the first patent reference, a motor and the like in the second patent reference, a drive with motor in the last patent reference) is fixed to the step plate. Therefore, the upper surface of the step plate (riding surface) is continuously parallel with a traveling surface (road surface). Accordingly, when the center of gravity is at a high position where a person is riding on the vehicle in a standing posture, an upper body of the rider is swayed right and left and becomes unstable by the action of the gravitational force during a travel on a cant road surface where the road surface inclines in the direction orthogonal to the traveling direction or by the action of the centrifugal force during making a turn. Accordingly, there is a possibility that the vehicle body falls in a lateral direction when such force becomes considerably large.
Details are explained in this regard by referring to FIGS. 1 through 3. FIGS. 1A through 1C are explanatory diagrams respectively showing a state of the coaxial two-wheel vehicle of related art viewed from the front side of the vehicle. FIGS. 1A through 1C show the whole of a coaxial two-wheel vehicle 1 including a vehicle body 2 also used as a step plate. Left and right wheels 3L and 3R are rotatably provided on both sides in the direction orthogonal to a traveling direction of the vehicle body 2. In addition, reference numeral 4 denotes a riding object (such as a person, for example) riding on the vehicle body 2, reference symbol G denotes the center of gravity of the rider 4, and reference symbol W denotes the weight (load) of the rider 4.
FIG. 1A shows a state of the coaxial two-wheel vehicle 1 traveling straight on a flat road surface without an influence of lateral force and centrifugal force. In this state, the center of gravity G of the rider 4 is positioned approximately above the center of the coaxial two-wheel vehicle 1 and the load W acts vertically to work at the approximately center of the vehicle body 2. Accordingly, approximately the same load acts on the left and right wheels 3L and 3R, and the reaction force thereof becomes approximately the same at ground contact points TL and TR where those wheels 3L and 3R contact with a road surface E.
FIG. 1B shows a state of the coaxial two-wheel vehicle 1 making a turn on the flat road surface E. In this state, centrifugal force (lateral force) F acts on the rider 4 from the right wheel 3R side, and a weight vector W of the load W slants by an angle θ due to the influence of the centrifugal force F. When a ground contact point R where an extended line of the weight vector W intersects the road surface E is inside the ground contact point TL of the left wheel 3L, the coaxial two-wheel vehicle 1 can make a turn with stability. However, when the ground contact point R shifts to the outside of the ground contact point TL as shown in FIG. 1C, the stability of the traveling may not be obtained because the left and right wheels 3L, 3R are unable to bear the centrifugal force F. Then, the coaxial two-wheel vehicle 1 may overturn (falling in the lateral direction) when the centrifugal force F that acts on the rider 4 becomes considerably large.
A difficulty level causing this coaxial two-wheel vehicle 1 to overturn greatly depends on the height of the center of gravity G of the rider 4. FIG. 2 is a diagram to explain the above. When the center of gravity G of the rider 4 is at a low position, a tilt angle allowed to the weight vector W of the center of gravity G is an angle θ as shown in FIG. 2. However, when the center of gravity G of the rider 4 is shifted to a high position, that is, to the center of gravity G1, the tilt angle at the center of gravity G1 becomes an angle θ1 which is smaller than the angle θ (θ1<θ), because a distance S from the center of the vehicle body 2 to the ground contact points TL and TR of the left and right wheel 3L and 3R remains unchanged.
From the above, the difficulty level of causing the coaxial two-wheel vehicle 1 to overturn is expressed by a product of the height of the center of gravity G and the centrifugal force F. Specifically, assuming that the ground contact point R of the weight vector W corresponds to the ground contact point TL of the left wheel 3L when the centrifugal force F acts on the center of gravity G, F×H=S (expression 1) can be obtained. Similarly, assuming that the ground contact point R of a weight vector W1 corresponds to the ground contact point TL of the left wheel 3L when a centrifugal force F1 acts on the center of gravity G1, F1×H1=S (expression 2) can be obtained. Accordingly, F×H=F1×H1. Here, F>F1 because H<H1. Therefore, when the center of gravity is positioned higher, the coaxial two-wheel vehicle 1 may overturn, even if the centrifugal force becomes smaller to that extent.
Such overturn of the coaxial two-wheel vehicle 1 can be prevented with a structure shown in FIG. 3. FIG. 3 is a diagram showing the vehicle body 2 being inclined toward the road surface E on the right wheel 3R side where the centrifugal force F acts. When the vehicle body 2 is thus inclined to the side where the centrifugal force F acts, overturn of the coaxial two-wheel vehicle 1 can be prevented and a stable turning can be made, because the ground contact point R of the weight vector W1 shifts to the inside of the ground contact point TL of the left wheel 3L.